Combining a coupled FTIR-EGA system and in situ DRIFTS for studying soil organic matter in arable soils

Abstract

An optimized spectroscopic method combining quantitative evolved gas analysis via Fourier transform infrared spectroscopy (FTIR-EGA) in combination with a qualitative in situ thermal reaction monitoring via diffuse reflectance Fourier transform infrared spectroscopy (in situ_<i>T</i> DRIFTS) is being proposed to rapidly characterize soil organic matter (SOM) to study its dynamics and stability. A thermal reaction chamber coupled with an infrared gas cell was used to study the pattern of thermal evolution of carbon dioxide (CO₂) in order to relate evolved gas (i.e., CO₂) to different qualities of SOM. Soil samples were taken from three different arable sites in Germany: (i) the Static Fertilization Experiment, Bad Lauchstädt (Chernozem), from treatments of farmyard manure (FYM), mineral fertilizer (NPK), their combination (FYM + NPK) and control without fertilizer inputs; (ii) Kraichgau; and (iii) Swabian Alb (Cambisols) areas, Southwest Germany. The two latter soils were further fractionated into particulate organic matter (POM), sand and stable aggregates (Sa + <i>A</i>), silt and clay (Si + C), and NaOCl oxidized Si + C (rSOC) to gain OM of different inferred stabilities; respiration was measured from fresh soil samples incubated at 20 °C and 50% water holding capacity for 490 days. A variable long path length gas cell was used to record the mid-infrared absorbance intensity of CO₂ (2400 to 2200 cm⁻¹) being evolved during soil heating from 25 to 700 °C with a heating rate of 68 °C min⁻¹ and holding time of 10 min at 700 °C. Separately, the heating chamber was placed in a diffuse reflectance chamber (DRIFTS) for measuring the mid-infrared absorbance of the soil sample during heating. Thermal stability of the bulk soils and fractions was measured via the temperature of maximum CO₂ evolution (CO₂_max). <br><br> Results indicated that the FYM + NPK and FYM treatments of the Chernozem soils had a lower CO₂_max as compared to both NPK and CON treatments. On average, CO₂_max of the Chernozem was much higher (447 °C) as compared to the Cambisol sites (Kraichgau 392 °C; Swabian Alb 384 °C). The POM fraction had the highest CO₂_max (477 °C), while rSOC had a first peak at 265 °C at both sites and a second peak at 392 °C for the Swabian Alb and 482 °C for the Kraichgau. The CO₂_max increased after 490 day incubation, while the C lost during incubation was derived from the whole temperature range but a relatively higher proportion from 200 to 350 °C. In situ_<i>T</i> DRIFTS measurements indicated decreases in vibrational intensities in the order of C-OH = unknown C vibration < C-H < −COO/C =C < C = C with increasing temperature, but interpretation of vibrational changes was complicated by changes in the spectra (i.e., overall vibrational intensity increased with temperature increase) of the sample during heating. The relative quality c...

Identifying and quantifying geogenic organic carbon in soils – the case of graphite

“…Oxidation of graphite upon heating could result in specific infrared absorption bands (Tan et al, 2013), which nevertheless would still be superimposed by SOM-specific bands in natural soil samples. As discussed by Demyan et al (2013), not only the available oxygen but also the heating rate has an important effect on the charring of OM and thereby on the thermal and spectral properties of the studied material.…”

“…An interesting observation was the similar predicted graphite content in calibration set 1 by both the FTIR and TGA methods, especially as FTIR is based on spectral properties and TGA on the thermal stability of the graphite. It has previously been suggested to combine FTIR and TGA systems to rapidly characterize the soil OM (Demyan et al, 2013). Oxidation of graphite upon heating could result in specific infrared absorption bands (Tan et al, 2013), which nevertheless would still be superimposed by SOM-specific bands in natural soil samples.…”

Identifying and quantifying geogenic organic carbon in soils – the case of graphite

“…Oxidation of graphite upon heating could result in specific infrared bands (Tan et al, 2013), which nevertheless would still be superimposed by SOM-specific bands in natural soil samples. As discussed by Demyan et al (2013), not only the available oxygen, but also the heating rate has an important effect on the charring of OM and thereby on the thermal and spectral properties of the studied material.…”

“…An interesting observation was the similar predicted graphite content in calibration set 1 by both the FTIR and TGA methods, especially as FTIR is based on spectral properties and TGA on the thermal stability of the graphite. It has previously been suggested to 20 combine FTIR and TGA systems to rapidly characterize the soil OM (Demyan et al, 2013). Oxidation of graphite upon heating could result in specific infrared bands (Tan et al, 2013), which nevertheless would still be superimposed by SOM-specific bands in natural soil samples.…”

DRIFTS band areas as measured pool size proxy to reduce parameter uncertainty in soil organic matter models

“…3). It was previously suggested that the high SOC storage capacity of this site is a result of cation-bridging due to a high content of adsorbed cations (Ellerbrock and Gerke, 2018). Additionally, there is evidence of black carbon at the site (e.g., the high thermal stability found by Demyan et al, 2013).…”